Experimental Investigation of the Positive 
After-Image in Audition 



BY 



HOMER GUY BISHOP 



Thesis presented to the Faculty of the Graduate School 
of Cornell University in partial fulfilment of the require- 
ments for the Degree of Doctor of Philosophy 



Reprinted from The American Journal of Psychology 
July, 1921, Vol. XXXII, pp. 305-325 



An Experimental Investigation of the Positive 
After-image in Audition 



BY 

HOMER GUY BISHOP 
if 



Thesis presented to the Faculty of the Graduate School 
of Cornell University in partial fulfilment of the require- 
ments for the Degree of Doctor of Philosophy 



Reprinted from The American Journal of Psychology 
July, 1921, Vol. XXXII, pp. 305-325 









o^P 



AN EXPERIMENTAL INVESTIGATION OF THE 
POSITIVE AFTER-IMAGE IN AUDITION 1 



By Homer Guy Bishop 



Contents 

Page 

Problem 305 

Historical 306 

Experimental: Series I. (Variators) 307 

Series II. (Variators) 310 

Series III. (Variators) 313 

Series IV. (Telephone Receivers) 314 

Significance of Results 315 

Experimental: Series V. (Telephone Receivers) 320 

Nature of the Modified Ending 321 

Experimental: Series VI. (Complex Tones) 323 

Recurrent Images 324 

Conclusion 325 

Our problem is to find out if there is a positive auditory 
after-image, analogous to the positive after-image in vision, 
and in that case to describe it in attributive terms. If we 
detect the presence of auditory experience after the stimulus 
ceases, we must seek to identify this experience as after- 
image, or memory after-image, or memory image, or what- 
ever it may be. 

1 From the Psychological Laboratory of Cornell University. 

305 



306 BISHOP 

Historical 

The first experiments upon the after-effect of auditory stimulation 
appear to have been made by Mayer. 2 The sources of tone were 
tuning-forks supplemented by resonators. The tones were conducted 
to the one ear by means of a rubber tube; the other ear was plugged 
with wax. Between the nipple of the resonator and the free end of 
the conduction-tube stood a siren disk. When a space between the 
holes in the disk was before the nipple, the sound waves were blocked ; 
when a hole was before the nipple, the sound passed into the tube at 
full intensity. As the disk was rotated, short periods of stimulation 
alternated with equal periods without stimulation. The critical value 
sought was the length of the interval between tones which would just 
be bridged, without loss of sensible intensity, by the after-effect, so 
that the O should hear a smooth tone. 

Urbantschitsch 3 worked by a similar method. The tones were in- 
terrupted by a pendulum, which carried the one end of a conduction- 
tube to and fro past the limbs of a Y-tube. The tube to O's ear, 
which completed the conduction system, was connected to the stem of 
the Y-tube. Unlike Mayer, Urbantschitsch sought to determine the 
critical interval at which the tones would just fall apart. The aim of 
the experiments was to measure the full duration of the after-effect, 
whereas Mayer measured only that part of it which showed no decrease 
of sensible intensity. 

Schaefer 4 criticizes the method of interruptions on the ground that 
it cannot take separate account of the Abklingen and Anklingen 
of the tones employed. Marbe 5 also considers the method inadequate. 
" An investigation of the facts of Abklingen in auditory sensation can 
not be made ... by means of successive, periodic stimuli, but only 
by the more difficult means of isolated auditory stimuli." 

Urbantschitsch 6 further studied what he calls " primary " and " sec- 
ondary " positive auditory after-images. The " primary " after-image 
is so closely joined to the sensation that there is no perceptible break 
between the two ; the " secondary " after-image follows the sensation 
only after an interval, and may recur several times, at intervals of 
varying length. Urbantschitsch's method is so imperfectly reported 
that it is impossible to undertake a repetition of his experiments. We 
are told only that three tuning-forks were sounded at four intensities 
(characterized as "very strong," "strong," "moderately strong," and 
" weak ") for periods of 5 and 15 sec. 

Experimental 

We employed two sources of tone : Stern variators, with 
tube-transmission, and vibrating telephone receivers. It may 
be said at once that the principal difficulty with either source 

2 A. M. Mayer, "Researches in Acoustics," Amer. Jour, of Science 
and Arts, 147, 1894, 1 ff. 

3 V. Urbantschitsch, " Ueber das An- und Abklingen acustischer 
Empfindungen," Archiv. fur d. gcs. Physiol, 25, 1881, 323 ff. 

4 K. L. Schaefer, Nagel's Handb. der Physiol, 3, 1905, 507. 

5 K. Marbe, " Akustische Priifung der Thatsachen des Talbotschen 
Gesetzes," Archiv fur d. gcs. Physiol, 100, 1903, 557. 

(i V. Urbantschitsch, op. cit.; and " Zur Lehre von der Schall- 
empfindung," Archiv fur d. gcs. Physiol, 24, 1881, 585 ff. 



THE POSITIVE AFTER-IMAGE IN AUDITION 307 

was to secure a clean cut-off of the stimulus-tone. Our suc- 
cessive arrangements, therefore, represent repeated efforts at 
improvement in this respct. The experiments were performed 
in a suite of three rooms. It was necessary to spread the 
apparatus over a wide space on account of the power of 
tones to penetrate even heavy stone walls. Accordingly the 
O's sat in one room, E operated the apparatus in another, and 
a large unoccupied room lay between. Even so we found that 
the sources of the higher pitches at greatest intensity must 
be enclosed in soundproof boxes. 

Series I 

In our first series of experiments the source of tone was the vari- 
ator. Four pitches were used, 1024, 512, 256, and 128 vs. The vari- 
ators were blown at three intensities, and for three durations of 
stimulation. The three intensities were obtained by setting the air- 
pressure at 15.0, 7.6, and 4.0 mm. of water for the pitch of 1024 vs ; 
at 5.0, 2.6, and 1.6 mm. for 512; at 19.0, 11.6, and 8.0 mm. for 256; 
and at 8.0, 5.5, and 4.0 mm. for 128. The nozzle of the variator was 
so adjusted to the mouth of the cylinder that a maximum of intensity 
should require the lowest possible pressure. We found that the pres- 
sures could not be equated, but varied from instrument to instrument, 
as indicated by the pressure-values just listed. The durations were 
5, 15, and 30 sec. read directly from a Pye clock. Since the seconds- 
hand of this instrument moves over a dial of 11 cm. diameter, the 
spaces between seconds are broad enough to admit of accurate reading. 

A special control enabled us to adjust the air-pressure quickly and 
accurately. A wooden lever 85 cm. long, screwed to the handle of the 
rotary valve, swung before a ruled pressure-scale. From one side of 
the lever and 60 cm. from the valve, two metal arms (2 cm. long and 
0.5 cm. broad) extended parallel to each other and at right angles to 
the edge of the lever. They were 4 cm. apart, and a fine wire was 
drawn taut from the outer end of the one to the outer end of the other. 
This wire, parallel to the edge of the lever and to the lines of the 
scale, lay on the surface of the scale in such wise that the error of 
parallax was avoided. 

The sound-waves were picked up by a funnel of cardboard and 
reflected into a brass conduction-tube of 1.9 cm. inside diam. and 
0.15 cm. thickness. The small end of the funnel fitted snugly over 
the end of the brass tube; the diameter of the large end was 15.0 cm.; 
and the length of an element in its surface was 45.0 cm. This slant 
is that of an old model of Victrola horn, and is approximately the 
same as the slope of the funnel in Marbe's Spr'achmelodie-Apparat. 
When the variator was set in position, its mouth was in line with the 
axis of the funnel, just not touching the edge. 

The brass conduction-tube was 8.76 m. long. It passed through two 
walls, across the middle room of the suite, and ended in O's room in 
an iron pipe 2.3 m. long. The iron pipe, which lay horizontally against 
the wall and at right angles to the brass tube, served as distributor of 
the tones to the four booths in which the O's sat. Four nipples were 
tapped into it, and from these four rubber tubes, of 0.6 cm. inside 
diam., led to four pairs of stethoscopic ear-pieces. The brass tube 



308 Bishop 

entered the iron pipe at its middle, and the nipples were at 36 cm. 
and 104 cm. right and left. Under these conditions, the intensity at 
all listening tubes was sensibly the same. The booths in which the 
O's sat were separated by heavy curtains. 

We have described above the valve which controlled the blowing 
pressure. It was necessary to introduce another valve between the 
pressure-valve and variator, which should cut off the air suddenly and 
bring the tone to a sharp ending. The special valve constructed for 
this purpose somewhat resembled the piston-valve of a cornet. In one 
position of the piston, the air went straight through the one opening 
to the variator, while the other opening was closed at the tube in 
which the piston moved. In the other position, the passage was re- 
versed ; the hole which had been open was now closed, and conversely. 
In this second position, no more air could enter the part of the tube 
between valve and variator, and the air which was present under full 
blowing pressure when the valve first closed had two outlets, the one 
by way of the nozzle of the variator, the other by way of the passage 
through the valve. Since the latter passage offered the lower resist- 
ance, most of the pressure was spent in this direction, and the terminal 
" whoop " which otherwise was very troublesome to the O's was 
reduced to the vanishing point. (Unless the air escaping from the 
tube, in the process of reduction of the pressure within it to atmos- 
pheric pressure, can be diverted from its course through the nozzle, 
the pitch of the tone in dying falls quite perceptibly as the pressure 
diminishes.) The diam. of the holes through the piston, 0.6 cm., was 
the same as the inside diam. of the rubber tube carrying the air to the 
valve ; so that the piston need be moved no more than 0.7 cm. to open 
the one passage and close the other. In the first series of experiments 
we threw the valve by hand. 

As warning signals we used four small 4-volt incandescent lamps 
placed upon the wall before the O's. The circuits were so arranged 
that the light could be made to glow at two intensities. The lamps 
were turned on at the lower intensity at the beginning of the experi- 
mental hour, and remained at this intensity throughout, except when 
they were flashed as signals ; at the lower intensity they showed merely 
a dull red glow. Our object was, by keeping the filament constantly 
warmed, to increase the sensitivity of the lamp. We needed to have 
it flash brightly in an instant, an impossible result if the filament were 
cold at the outset. E closed a key to flash the usual " Ready," " Now " 
signals. When the air was turned on at the valve, the same movement 
closed a mercury-contact key attached to the piston of the piston- 
valve, and the lights burned brightly. When the air was cut off, the 
movement of the piston broke the circuit, and the lights dropped to the 
lower intensity. This method secured temporal coincidence of the 
cutting-off of the tone and the dimming of the lamp. 

Every O had at his hand a key making or breaking the circuit in 
the recording apparatus. This apparatus consisted of eosin writers 
adapted to use with ticker paper. They were made of tin; and though 
different in structure their principle of operation was that of the 
draughtsman's ruling pen. They wrote from the horizontal position, 
withstood hard knocks, and did not spill the fluid when tapped rapidly 
against the paper. When O's key was closed, the tip of the writer 
was pulled against the paper by an electromagnet, and a line was 
written representing the length of any after-effect which might be 
experienced. When O was ready for the next tone he tapped the key 
as a signal to E. The writers were so slender that we were able to 



THE POSITIVE AFTER-IMAGE IN AUDITION 309 

write one time-line and four record-lines upon the usual ticker paper 
1.8 cm. in width. 

The dashes in the time-line were made by a Kronecker interrupter 
vibrating in tenths of sec. To fix the exact moment at which the tone 
began, we resorted to another mercury-contact key upon the piston of 
the piston-valve. When the air was cut off, the circuit was closed, 
and the time-writer wrote a continuous line; when the piston moved 
to turn on the air, the circuit was broken, and under power from 
another circuit the interrupter-point began to write tenths of a sec. 
The paper was drawn by a motor which ran throughout the experi- 
mental hour; but the feed of the paper was controlled by a clutch, 
and it was drawn past the writer only when needed. With this ar- 
rangement there was no lag of the paper; it was drawn immediately 
at full speed. 

Observers. The O's were Mr. R. T. Holland, graduate stu- 
dent and assistant in the department; Miss M. F. Martin, 
Dr. C. W. Perky, Miss A. H. Sullivan, and Mr. S. Takaki, 
graduate students majoring in Psychology. Observer P was 
somewhat more highly practised than the others, but all were 
experienced O's. 

Instructions. " You will hear a tone which will begin when 
the signal light brightens, and will cease abruptly as the light 
dims. You are to note whether the tonal sensation continues 
or recurs after the dimming of the light. If it continues, you 
are to press the key immediately and release it at its final 
disappearance. If it recurs, you are to press the key at every 
appearance and hold it until the disappearance of the recurring 
sensation. When the experiment is over, you will write a 
descriptive account of the after-sensation or after-sensations 
on the blanks provided." 

For every pitch there were 9 stimulations presented in the 
order : strong-short ; middle intensity-middle duration ; weak- 
long; strong-middle duration; strong-long; middle intensity- 
short ; middle intensity-long ; weak-short ; weak-middle dura- 
tion. This series of 9 stimulations was run off first with the 
highest pitch, then with the remaining three pitches in descend- 
ing order. There were thus 36 experiments in a series. Every 
series was repeated five times, giving a grand total of 180 
experiments for every O with this first arrangement of ap- 
paratus. 

Results 

Our results showed a considerable number of cases in which 
the tones " ended abruptly," were " chopped off," with no trace 
of terminal modification or after-effect. Besides these, how- 
ever, there were cases described in which the ending of the 
tone was modified, in quality, in intensity, or in temporal 



310 



BISHOP 



course. The O's spoke of " slow dying," " prolonged ending," 
" not abrupt," a " whoop." Never did the modified ending 
carry sensibly beyond the dimming of the light, or suggest 
to O the continuation of the sensation in a positive after-image. 
Table I is a summary of all modified endings. Unfor- 
tunately, we cannot be sure of their interpretation. The slow 
cooling of the filament in the lamp may have led to error in 
judgment on the part of the O's. The apparatus could not 
guarantee that what was heard was the natural ending of a 
suddenly stopped tone ; so that these modified endings may 

TABLE I 





Intensity- 


Duration 


Pitch 


Total 





Strong 


Medium Weak 


30 15 5 


1024 


512 


256 


128 


H 
M 
P 
S 
T 


22 
30 
36 

37 
30 


18 7 
27 29 
29 22 
32 38 
25 20 


17 14 16 
29 32 25 
27 30 30 
39 38 30 
26 26 23 


18 
28 
24 
35 
22 


14 
20 
23 
34 
19 


14 
20 
27 
16 
23 


1 
18 
13 
20 
11 


47 
86 
87 
107 
75 


Total 


155 


131 116 


138 140 124 


129 


110 


100 


63 


402 



in fact have followed the last vibration, and thus may repre- 
sent an after-effect. Besides this serious defect in the appa- 
ratus, our first series of experiments indicated that several 
minor changes were necessary. Noises in E's room reached 
the O's through funnel and conduction tube ; the purr of the 
motor which drew the ticker paper had in it a note that was 
easily mistaken for an after-image; the hand-operated valve 
was not always thrown with equal speed from the one position 
to the other; and any echo of the tones in E's room might 
reach the O's. 



Experimental: Series II 

We chose the lights as signals because they were silent; but they 
were not reliable; and we now substituted for them small electro- 
magnets from a player-piano. These magnets are not noisy ; and 
when operated by a weak current, just strong enough to produce a 
light, quick movement, they did not annoy the O's. To the armature 
of the magnet we soldered a strip of tin, 4 cm. by 0.5 cm., to serve as 
signal flag. Every O had a magnet and flag in his booth before him, 
at the place where the light had been. When the current was off, the 
flag stood almost vertically ; when it was turned on, the flag was 
pulled quickly down to the horizontal. 

In order to operate the piston-valve quickly and at a constant speed 
at all times, we built two solenoids end to end on a single base. They 
were mounted at the end of the piston, and an extension of the piston 
passed through them both. One solenoid pulled the piston to turn the 
air on, the other pulled it back again to turn the air off. With a suffi- 
cient current the shift was almost instantaneous. 



THE POSITIVE AFTER-IMAGE IN AUDITION 311 

We shut out the noises from E's room by placing the variators in 
a sound-proof box. We cut the funnel down to a smaller size (diam. 
10 cm., length 25 cm.) for use inside the box. The whole conduction 
system was now isolated, though there was still the possibility of echo 
from the walls of the box. Indeed, we found that there was a faint 
tone which made its appearance regularly at a short interval after the 
air was cut off. We satisfied ourselves that this tone was in the ap- 
paratus, and was not subjective, by making records of the vibrations. 
A diaphragm of the Marbe Sprachmelodie-Apparat was held firmly 
against O's end of the conduction tube, and the smoke-rings regis- 
tered the tone. 

To rid ourselves of this source of error, we so hinged one of the 
sections of brass conduction-tube that the one end of it could be drawn 
aside. The other end was attached by a heavy rubber tube to the fixed 
end of the next section of conduction-tube. An electromagnet in series 
with the solenoid which cut off the air to the variator pulled this 
swinging section of tubing to one side at the same time that the vari- 
ator ceased to be blown. In this way the echo was diverted into the 
middle room of our suite, and the O's could not hear it. We found 
it very difficult to swing the pipe noiselessly, but we finally hit upon 
the following arrangement. One end of a thin strip of wood, about 
1 m. long, was fastened to the floor with a hinge ; the other end carried 
the swinging end of the conduction-tube. The electromagnet was fas- 
tened to a firm support about 30 cm. from the floor. A piece of iron, 
screwed to the thin strip at this level, was attracted by the magnet, and 
the pipe was thus drawn aside. The iron was faced with felt ; and 
by this means, supplemented by the elasticity of the strip of wood, the 
pipe was brought to rest silently in its new position. The supporting 
wooden strip stood so nearly vertical that it was easily pulled toward 
the magnet ; but it was inclined so far away from the vertical that 
gravity pulled it back to the first position when the circuit to the 
magnet was broken. A piece of spring steel, faced with felt where 
it touched the wooden strip, furnished a silent stop for the pipe when 
it returned to the position for conduction. The swinging section was 
1.82 m. long; with a radius so large, and an outside diam. of the tube 
of only 2.2 cm., the space between the swinging end and the fixed end 
of the next section could be about 0.25 mm. The space was adjusted 
at every experimental hour so that the ends should just not touch. 
The break in the pipe was 84 cm. from the point at which the brass 
tube joined the iron pipe in O's room. We felt satisfied that, with a 
distance so short as this, no residual sound-waves remaining in, the 
conduction system between the O's and the break in the tube could 
have any effect for audition. 

In the light of our first results, it is clear that the warning signal 
must be very accurately timed. The modification in the ending of the 
tone was so short that the flag must move in exact coincidence with 
the last sound-wave. We turned again to the Marbe apparatus. It 
was set up with two diaphragms and one gas-burner, all in the same 
gas system. One diaphragm made the tone-rings, the other the ring 
from the flag. The diaphragm at the flag was brought so near the 
flag that the first beginning of movement was recorded by the mem- 
brane. By this arrangement the effects from the two membranes were 
superimposed. The record should show a band of regular rings repre- 
senting the tone, and then an aperiodic ring written by the flag, fol- 
lowed by a band with no rings. By carefully adjusting the contact 
placed where the swinging section of conduction-tube would close the 



312 



BISHOP 



circuit to the flags, we were able to secure this kind of record. The 
heavy ring from the flag showed as the last ring in the series, and it 
stood at the proper distance from the last ring but one to be in coin- 
cidence with the last tone-ring. To be sure, the record did not indicate 
whether the heavy ring was the result of a summation of two rings ; 
but it did show that the flag was not more than one vibration out of 
step with the tone (256 vs.). Still, however, there was the possibility 
that the stroke of the flag blotted out the tone-rings ; tone might be 
present, but unrecorded. We put this possibility to the test by allow- 
ing the flag to make its ring without cutting off the tone. We found 
that the tone waves were just as distinct in the record after the stroke 
of the flag as they were before. The disturbance in the flame set up 
by the flag did not blot out the effect of the tone, but gave a super- 
posed effect. 

With these improvements in the apparatus we were ready for the 
next set of experiments. 

Observers. The O's were H, M, S, T of the previous group. 

Instructions. " You will hear a tone which will begin when 
the signal flag falls, and will continue for a varying period 
of time. About a second before the tone ends, the flag will 
fall as a warning signal that the tone is about to end. Just 
at the instant that the tone ceases abruptly, the flag will 
fall again. You are to note whether the tonal sensation con- 
tinues or recurs after the flag falls. If it continues, you are 
to press the key immediately and release it at the final dis- 
appearance of the after-sensation. If it reappears, you are 
to press the key at every appearance and hold it until the 
disappearance of the recurring sensation. When the experi- 
ment is over, write a descriptive account of the after-sensa- 
tion or after-sensations on the blanks provided." 

The experiments in this series were conducted with the 
same tones as before, in the same order. Except for the 
changes in the apparatus, already described, and for the warn- 
ing signal given during the course of the tone, the method is 
the same as in the first set of experiments. 

Results 

The distribution of the modified endings is shown in Table 
II. The decrease in number between Series I and Series II 

TABLE II 





Intensity 


Duration 


Pitch 


Total 





Strong 


Medium 


Weak 


30 15 5 


1024 


512 256 


128 


H 

S 
T 
M 


5 

35 
6 


1 

28 

2 

(none) 




26 

2 


2 3 1 

36 32 21 

7 3 

(none) 


2 
32 

4 


4 

30 18 

2 

(none) 



9 

4 


6 
89 

10 


Total 


46 


31 


28 


45 38 22 


38 


32 22 


13 


105 



THE POSITIVE AFTER-IMAGE IN AUDITION 313 

seemed to indicate that further refinements might lead to their 
complete elimination. We, therefore made additional efforts 
to refine our technique. 

Experimental: Series III 

So long as the tones were stopped by cutting off the air, there was 
escape of the air under pressure between the nozzle of the variator 
and the piston-valve. An unknown part of this air must certainly- 
escape by way of the nozzle; and its escape, if audible, might lead to 
O's reports of modified endings. 

Only after we had discovered how difficult it is to avoid the influ- 
ence of the diminishing air pressure, did we turn to the rather obvious 
plan of allowing the variator to blow continuously, while we broke 
the path of conduction in order to obtain the periods of silence between 
stimulations. We accomplished this by swinging more sections of the 
conduction-tube. The section at the sound-proof box was pivoted at 
the middle to allow the ends to move up and down. The fulcrum was 
a wad of cotton, packed rather tightly around the pipe in the hole in 
the stone wall through which it passed. The next section was joined 
to this lever of conduction-tube and to the third section in the line 
by rubber tubing fitted over the ends of the pipe. When the end of 
the pipe in E's room was thrown down, the end in the middle room 
went up, carrying with it the end of the second section. This move- 
ment produced a break in the line at the box, but preserved the con- 
tinuity in the next room. At the other side of the room was the sec- 
tion adjusted to swing in the previous set of experiments. The swing- 
ing end was now supported by a wire and pulley, so that it could be 
drawn up and dropped down. The means of obtaining simultaneous 
and equal movement at both breaks in the conduction system was very 
simple. The wire just mentioned extended across the middle room; 
its ends dropped vertically down from the pulleys and were fastened 
to the conduction-tube, the one end to the end of the lever section, 
the other to the free end of the swinging section. When the end of 
the lever at the variators went down, the end in the next room went 
up by an equal amount, since the fulcrum was in the middle. This 
upward movement allowed the wire to roll over the pulleys, and the 
end across the room dropped by an amount equal to the movement of 
either end of the lever section. The displacement thus secured was 
about 5 cm. The pull upon the pipe was exerted by means of a sole- 
noid fastened to the floor in E's room. A cord tied to the end of the 
lever-section supported the core at a good pulling distance above the 
center of the solenoid. With a strong current, the displacement was 
sudden and free from noise. We found it necessary, however, to sew 
the core of the solenoid into a close fitting bag, in order to secure 
silent action of the core within the coil. 

It is obvious that the shape of the ends of the conduction-tube at 
the break could not be circular. The diminishing amount of area for 
conduction as the ends became separated might be responsible for an 
observable decrease in intensity of the tone. The ends of the tube, 
except for a slit of 1.8 cm. by 0.3 cm., were accordingly plugged with 
sealing wax. The long dimension of the slits was horizontal, so that 
a quick movement of the end of the pipe downward brought them 
wide apart, too rapidly for any sensible decrease in intensity. We 
tried at first to use four of these slits, but found that they reduced 



314 



BISHOP 



the intensity; two at the sound-proof box were sufficient. The circuit 
driving the signal flag was again closed by the swinging pipe. 

We found that the two high tones could be heard three rooms away, 
even though the variators were in a box with very heavy double walls, 
if they were sounded at more than our intermediate intensity. We 
accordingly decreased the blowing pressure to 2, 4, 7.6 cm. of water 
for the tone of 1024 vs.; and to 0.9, 1.6, and 2.6 cm. of water for 
that of 512 vs. The two breaks in the conduction-tube weakened the 
low variator tones beyond usable limits. In their stead we used tun- 
ing-forks (with resonators) of 256 and 128 vs. The resistance in the 
circuit was such that at 10 v. we obtained satisfactory driving power 
for the forks at 1, 2, and 3 amps, for 256 vs. ; and at 2, 3, and 5 amps, 
for 128 vs. 

Results 

The results from this series of experiments are shown in 
Table III. We reserve comment upon them until later. 

TABLE III 





Intensity 


Duration 


Pitch 


Total 





Strong 


Medium 


Weak 


30 15 5 


1024 


512 256 


128 


s 

T 


30 
1 


26 
3 


20 

4 


25 27 24 
3 3 2 


30 
3 


33 7 
3 1 


6 
1 


76 
8 


Total 


31 


29 


24 


28 30 26 


33 


36 8 


7 


84 



Experimented: Series IV 

The apparatus just described was satisfactory except for the fact 
that the tones were weak. As it stood we could be reasonably certain 
that the O's heard the natural ending of a tone, the sensible process 
of transition from tone to silence. The noise incident upon swinging 
the pipes was, it is true, barely noticeable just after the ending of the 
tone was heard, and a very faint after-image might have been lost. 
But the main fault lay with the intensity of the stimuli themselves. 
We required a source which should have fairly intense tones, and 
which should be completely silent in the period following the cessa- 
tion. After a good deal of cut-and-try experimenting, we had re- 
course to telephone receivers. 

The instrument was the ordinary variety of receiver of 75 ohms 
resistance. It was actuated by a Petzold double induction coil, with 
a current of 2 amp. at 6 v. through the two primaries connected in 
series. The vibration frequency in the receivers was determined by 
the number of interruptions in the primary, and not by a separate 
source of sound. By this method we avoided the complications which 
a transmitter would have introduced. The interrupters were tuning- 
forks of 435, 350, 256, and 100 vs. We were unable to drive the forks 
satisfactorily by the same current which passed through the primaries 
of the coil; by means of an extra pair of contacts put upon the forks, 
we passed two separate currents through them, the one driving the 
fork, the other leading to the induction coil. A condenser connected 
across the point of interruption in the circuit to the coil kept down 
the sparks at the interrupter, and gave a tone free from the usual 
click and buzz of receivers. 



THE POSITIVE AFTER-IMAGE IN AUDITION 



315 



The line across which the receivers were connected derived its power 
from the secondary coils connected in series. A special switch with 
four mercury contacts closed or opened the line to the receivers, by 
breaking both sides of the line at once. When one side alone was 
broken, the receivers sounded faintly; and they could be made silent 
only by interrupting both sides of the line together. This same switch 
also carried contacts controlling the signal flags. When the path to the 
receivers was broken, the circuit to the flags was made, just far enough 
in advance to counteract the slight lag in the flags. 

The three intensities of stimulation were obtained by setting the 
secondary coils both at 0; the one at 14, the other at 2; and both at 
14. There was no further change in the experimental procedure. 

Results 

Table IV contains a summary of modified endings from 
Series IV of 360 observations for every O. 

TABLE IV 





Intensity- 


Duration 


Pitch 


Total 





Strong 


Medium 


Weak 


30 15 5 


1024 


512 256 


128 


H 

S 
T 


120 

119 

6 


120 

120 

2 




113 

1 


80 80 80 

120 118 114 

2 3 4 


60 
90 

1 


60 60 
89 87 

4 


60 
86 

4 


240 

352 

9 


Total 


245 


242 


114 


202 201 198 


151 


149 151 


150 


601 



Significance 'of Results 

So far as concerns a positive after-image, analogous to the 
positive after-image in vision, we have nothing to report. In 
a total of 3,780 observations, the results have been flatly 
negative. But our ' modified ' endings are positive results ; 
and as they have been ineradicable, we are obliged to discuss 
them. 

The instructions laid emphasis upon the positive after- 
effects of our stimuli ; and the different O's took different atti- 
tudes toward the endings of the tones. In Series I and IV, 
H reported the modified endings frequently; in Series II, he 
was especially attentive to after-effects, and only the more 
pronounced of the modified endings were reported; in Series 
III, he made the general statement that he could hear a " ylup," 
if he made an effort to hear it, coincident with the drop of the 
flag. Throughout the series he took the instructions literally, 
and reported " Nothing " for the after-effect of every stimu- 
lus. M never reported modified endings after Series I, and 
like H never heard an after-effect. T reported modified end- 
ings very infrequently after Series I, but he never totally 
ignored them. S failed always to hear after-effects, but she 
persistently gave her attention to modified endings. 



316 BISHOP 

An examination of the totals in Tables I to IV shows that 
for every series the frequency of the report of modified end- 
ings is directly proportional to the degree of intensity of 
stimulus. Even in Series III, where the strongest stimulus 
was rather weak, the rule holds in some measure. The in- 
fluence of duration is less regular ; but there are always fewer 
modified endings reported at 5 sec. than at 15 and 30 sec. 
In the case of pitch, the tables may be misleading. In Series 
I and II, decrease in frequency of modified endings parallels 
a fall in pitch; but the high tones were intense and the low 
were weak; so that what might be taken for an effect of 
pitch is in fact an effect of intensity. We feel assured, from 
the results in Series III and IV, that this interpretation is 
correct. In both these series all pitches were of nearly equal 
intensity, and the distribution of modified endings is also 
nearly equal for all pitches. The contradiction in Table III 
is only apparent. It was very difficult to keep the tuning- 
forks of 256 and 128 vs. vibrating at low voltages, and for 
this reason we gave only one series with each. At higher volt- 
ages the forks had so great an amplitude of vibration that they 
struck the core of the electromagnet ; for this reason it was 
impossible to grade the intensities from a higher maximum. 

Throughout the earlier series, we regarded the modified 
ending as due to a fault in technique. We believed that re- 
finement of apparatus would remove it. The agreement be- 
tween number of modified endings and intensity of stimulus 
supported this opinion; but me influence of duration was 
against it. In order to explain the effect of duration, we 
searched the results to find what terms were used in describ- 
ing the modified ending. Many references to it were not 
descriptions, but characterizations like " swoop," " whoop," 
" sigh," " groan," etc. In Series III, H and S agreed that the 
difference between the tone and its ending was, in part, one 
of ' vocality/ which changed as the tone went off. S further 
described the ending as having " less body than the tone ;" it 
" disappeared out in space," " decreased in volume," " thinned 
out," " lost quality," etc. We have set down the attributive 
conditions of stimulus under which such descriptive terms were 
used in Table V. The results are all taken from Series III, 
since it is in this series only that the O's maintained a con- 
stantly descriptive attitude. 

The uniform distribution for duration shows that the modi- 
fied ending is not dependent upon differences in that attribute 
of stimulus. 



THE POSITIVE AFTER-IMAGE IN AUDITION 317 

TABLE V 





Intensity 









Strong 


Medium 


Weak 


H 


120 




120 





S 


37 




41 


43 




Duration 









30 




15 


5 


H 


80 




80 


80 


S 


40 




41 


40 




Pitch 









435 


350 


256 


100 


H 


60 


60 


60 


60 


S 


33 


42 


26 


20 



In the same group of experiments, certain terms indicative 
of judgments of intensity, like " pronounced swoop " and " less 
uh," were used. Their distribution is shown in Table VI. 





TABLE VI 








Intensity 









Strong Medium 


Weak 


H 

S 


120 
10 

Duration 


120 
5 



3 



H 

S 


30 

80 

6 

Pitch 


15 

80 

6 


5 

80 
6 



H 

S 


435 350 
60 60 
10 2 


256 

60 

1 


100 

60 

5 



The extreme regularity of H's results is due to a ' set ' for 
intensity. He reported only upon the relative intensity of the 
modified endings, which he found to be well above the limen. 
The figures show again that the modified endings are inde- 
pendent of duration. In Series I, where S frequently gave 
judgments of length of modified ending, the same result was 
found : the modified ending was short for all durations of 
stimulus. 



318 BISHOP 

It will be remembered that in every series every degree of 
intensity was given once with every duration of stimulus. In 
Series III, we counted the number of times that S used the 
same term to describe the modified ending for all three dura- 
tions in a series or for all three intensities. The number of 
cases is shown in Table VII. 





TABLE VII 






Intensity 




Strong 
12 


Medium Weak 
8 1 

Duration 


Total 
21 


30 

1 


15 5 
2 2 

Pitch 


Total 
5 


435 
9 


350 256 
6 2 


100 

9 



It is evident that duration does not markedly affect the 
modified ending, seeing that for 12 of the 15 times that the 
strong stimulus was given O is able to call the effect by the 
same name in spite of the 3 different durations of stimulus. 
The single case recorded in column 3 is very misleading in 
that 7 of the 12 weak stimuli were negative, i.e., ended abruptly 
for sensation, leaving only 5 times that a single term could 
be used. The table shows that duration had a slight influ- 
ence in determining the descriptive term ; but it is only about 
one quarter as effective as intensity. The 7 negative cases 
were distributed, 5 to the short, and 2 to the intermediate 
duration. 

In Tables I to IV, difference of duration appears to be 
effective in determining the number of reports of modified 
ending, and may, therefore, be a factor in determining their 
existence or non-existence. In Tables V to VII, duration 
does not modify the character of modified endings. Our 
experiments offered the suggestion that pressure in the ears 7 
may have been of importance in forcing attention to the 
modified ending. T reported pressure or deafness following 
the stimulus under conditions which are shown in Table VIII. 

7 On the perception of silence, see E. B. Titchener, " A Further 
Word on Black," Jour, of Philos. Psychol, and Sclent. Methods, 13, 
1916, 649 ff. 



THE POSITIVE AFTER-IMAGE IN AUDITION 319 

These results are all taken from Series IV, in which the 
stimuli were sufficiently strong to make pressure and deafness 
moderately conspicuous. We conclude from them that both 
intensity and duration co-operate to bring about a state called 
sometimes deafness, sometimes pressure. The only time that 
a weak stimulus produced pressure was when its duration 
was 30 sec. Here, then, is an effect of duration which may 
be responsible for its otherwise unexplained influence upon 
the frequency of report of modified endings. Through it, 
only those modified endings will be reported which force them- 
selves to the focus of attention. Our instructions, however, 
tended to direct attention to the period following the ending 
of the tone. Hence we should expect a report of pronounced 
or conspicuous endings, but of no others. 







TABLE VIII 








Intensity 






trong 
19 
4 
23 


Medium 

18 

7 

25 


Weak 
1 

1 


Pressure 
Deafness 
Total 






Duration 




30 

18 

6 

24 




15 
12 

2 

14 


Pitch 


5 

8 

3 

11 


Pressure 
Deafness 
Total 


435 

19 

4 

23 


350 
4 
3 

7 




256 

9 

4 

13 


100 
6 

6 


Pressure 
Deafness 
Total 



In order to substantiate this explanation, it becomes neces- 
sary to show, by a special direction of attention, that modified 
endings are, in reality, always present. H had already said 
that he could hear them with every stimulus, if he tried. Prof. 
H. P. Weld (W), and Dr. L. B. Hoisington (Ho.), members 
of the department and highly experienced O's, had previously 
been called in for supplementary observations following Series 
III. At first, they reported modified endings, but when they 
assumed a passive attitude toward them, and directed their 
attention to the silence following the end of the tone, the 
number of modified endings observed became fewer. Their 
reports indicated, nevertheless, a possibility that all endings 
of tone are modified. 



320 BISHOP 

To put the matter to the test, we made a few experiments, 
with Dr. K. M. Dallenbach, instructor in the department, H, 
Ho, M, T, and B (the writer) as O's. 

Experimental: Series V 

The stem of the Y-tube at the ear-pieces was held in the left hand ; 
and the end of the rubber tube, which previously had been joined to 
the Y-tube, was held in the right. Both free ends were brought to- 
gether, just not touching, for stimulation, and were suddenly drawn 
apart to remove the stimulus tone. The movement was made in all 
possible directions, with equal effect upon the auditory experience; 
and the thumb held against the end of the rubber tube to cut off the 
tone had no different result from that noted with movement of the 
tube. These movements provided a soundless means of discontinuing 
stimulation. O was asked, first, to direct his attention to the change 
in the character of the tone just as it ended, and to report whether it 
could always be heard ; secondly, to turn his attention to the period 
of silence following the tone, and to report, from stimulation to stimu- 
lation, whether the silence seemed more or less distinct from time to 
time ; and thirdly, to report the times when the modified ending espe- 
cially attracted attention. Stimulations were given for 5, 15, and 30 
sec. 

Results 

Every O was able to hear a modified ending of every stimu- 
lation, if attention were directed upon it, even with weak 
tones carried almost to the limen. Every O found that the 
stimulus of 5 sec. was incapable of producing any but the faint- 
est pressure which is characteristic of perceiving silence. Dura- 
tions of 15 and 30 sec. definitely produced the pressure; but 
the O's did not agree whether 30 or 15 sec. produced the 
greater pressure. Every found also that the modified end- 
ing was not insistent after the 5 sec. stimulation, but that it 
stood out very conspicuously from stimulations of 15 and 30 
sec. In B's experience the heaviness of the deeper silence from 
long stimulation worked, as if by contrast, to make the modi- 
fied endings stand out more vividly without necessarily becom- 
ing more intense. It seemed to be the ' contrast ' between 
silence and modified ending which grew more intense as the 
stimulus-time was lengthened. Since the pressure is cumu- 
lative with the product of intensity and duration of stimulus ; 
and since the intensity of the modified ending seems to depend 
upon intensity of stimulus ; we conclude that the enhanced 
' contrast ' effect comes by way of increased pressure. 

This, then, seems to be the explanation of the apparently 
contradictory results. The modified ending varies in intensity 
directly with intensity of stimulus, and is always present. 
When the O's report abrupt endings, or fail to report the end- 



The positive after-image in audition 321 

ing at all, their direction of attention to the period of silence 
is complete ; and no endings, except the most vivid, come to 
their notice. Long stimulations enhance the contrast-effect 
between tone and silence, and increase the number of times 
that hears the modified ending; they thus give the im- 
pression that the longer the stimulus-time, the more frequent 
is the modified ending. 



Nature of the Modified Ending 

It is tempting to regard the modified ending as ' subjective ' 
in nature, and to identify it with the Abklingen 8 or ' drop ' of 
auditory sensation which Mayer and others have attempted 
to measure. This view is supported by the facts that the 
modified ending varies directly with intensity of stimulus, and 
that it shows always a thinning-out or reduction of the ' body ' 
of the tonal complex. While, however, we have no wish to 
dispute the presence of an Abklingen, we are nevertheless of 
the opinion that, in our experiments, this subjective factor 
was outweighed by objective changes. 

In the first place, we mistrust the results from the tele- 
phones. So long as the tone is stopped by taking away the 
driving power of the diaphragm, there is a source of error 
remaining : the diaphragm must come to rest from the position 
in which it happens to be when the current is cut off, and 
this return is definitely audible. Our arrangement with in- 
duction coil and condenser served to eliminate the noisy click 
natural to the instrument at the moment when it ceases to be 
under the influence of the electromagnet; but it is evident 
that the very fact of vibration still necessitates a return to a 
state of equilibrium. 

For a similar reason, the results from the variators are not 
free from objection. When the variator ceases to blow, we 
have no means of knowing through what stages the physical 
stimulus passes in coming to rest. At one moment the air 
is in motion, and a moment later it comes to rest. Our modified 
endings may depend in part upon objective change in the 
sound waves. When the path of conduction is broken by 
pulling the conduction-tube aside, eddy-currents must be set 
up around the ends of the tubes ; and these, though not audible 
at low intensities, may change the character of the stronger 
tones as they end. 

8 For Abklingen in general see C. Stumpf, Tonpsychologie, 1883-90, 
references in index: K. L. Schaefer, Nagel's Handb. der Physiol, iii., 
1905, 504 ft. 



322 bishop 

There are, indeed, positive indications that the modified 
ending is in part objective. (1) The results from Series I-III 
differ widely with respect to the number of times the pitch 
changed in any direction. A summary is given in Table IX. 

TABLE IX 

Series 

I II III 

Pitch rises 6 25 48 

" the same 48 15 23 

" falls 11 7 5 

Total 65 47 76 

In Series I, the vibration in the air spent itself normally. 
In Series II, when the pipe was drawn aside, the path of 
conduction was interrupted; and eddy-currents at the break 
may have changed the character of the ending of the tone. 
The ends of the tube were not plugged; and even though the 
pipe moved quickly, a certain brief time was required for the 
movement. In Series III, the ends were plugged, and the 
movement of the pipe was quicker than in Series II. The 
eddy-currents would be more intense, owing to the greater 
speed with which the pipe moved and to the narrowed opening 
in the pipe. Table IX shows that the increase in the number 
of judgments of higher pitch, and the decrease in the number 
of lower pitch, are correlated with the suddenness of the end- 
ing of the tone and the violence of the movement of the pipe. 
(2) We made a few experiments as a check upon the impli- 
cation of Table IX by the method of pulling the rubber tube 
away from the stem of the Y-tube at the ear-pieces. When 
the end of the rubber tube vibrates rapidly before the end 
of the Y-tube, the movement produces a distinct thud or noisy 
puff. If the movement be made sufficiently violent, the noise 
covers up the tone. B, H, Ho, M, and T all reported that 
the modified ending was most intense, at any intensity of 
stimulus, when the rubber tube vibrated 4 to 6 times per sec. 
This result proves that long stimulation does not add to the 
intensity of the modified ending, but that rate of vibration 
does, probably from the increase in energy of the stimulus 
due to interference of air-waves at the ends of the tubes. (3) 
B and Ho made a number of observations to determine the 
pitch of the modified ending, and found that it is dependent 
upon the rate of movement of the end of the tube. A quick 
movement gives an " oop " higher in pitch than the tone ; and 
a slower movement gives the same kind of sound, but its 
pitch is lower, and it is softer and has greater volume. Certain 



THE POSITIVE AFTER-IMAGE IN AUDITION 323 

rates of movement give a pitch that is equal to that of the 
generator. Both forks and variators were used as stimuli. 
If these observations, made under loose experimental condi- 
tions with hand-control of the speed of movement, may be 
trusted, it is possible that we have an explanation of the re- 
sults in Table IX ; for the judgment of pitch of the modified 
ending is correlated with the objective suddenness of the 
movement which interrupted the tones. The observations 
also furnish additional evidence that the modified ending may 
be objective; 9 and that, if a means of interrupting tones could 
be found which did not affect the air in the path of conduc- 
tion, it might in so far be eliminated. It may, however, be 
impossible to deprive a tone of its normal means of propa- 
gation, the air, without at the same time setting up disturb- 
ances in the air. 

Such results as ours call for further experiments conducted 
under the best possible conditions. As our problem was to 
demonstrate the presence or absence of an auditory positive 
after-image, the modified endings are incidental to our purpose ; 
and it is unnecessary that we should make a more complete 
investigation of the conditions upon which they depend. Since, 
however, their occurence, in so far as it represents an objec- 
tive disturbance of stimulus at cessation, might give ground 
for the objection that a possible faint after-image has been 
covered up and overborne, and thus has escaped the notice of 
our O's, we have made some further experiments with tones 
of greater intensity and longer duration. 

Experimental: Complex Tones 

In order to extend our results to other kinds of stimuli and to long 
stimulation-times, we conducted a group of experiments with more 
complex tones. In some experiments the original stimulation-times 
were used, and in others the time was extended to 5 and 15 min. The 
tones were C and C" from organ pipes of metal ; C and C from 
organ pipes of wood; and tones number 2, 4, 6, 8, and 10 from an 
Appunn reedbox of the overtones of a fundamental of 64 vs. 

Our O's in this group were Miss E. C. Comstock (C), graduate 
student in psychology; Mr. F. L. Dimmick (D), assistant in the 
department ;H, Ho, M, S, T; Mrs. A. K. Whitchurch (Wh), gradu- 
ate student .in psychology; and a class of 7 students in the advanced 
laboratory course. 

9 It was noticeable from the very first that O's who were observing 
in the same group did not by any means necessarily report in the same 
way upon the same stimuli. We have already shown that they varied 
in their attitude, attending sometimes to the tone's ending and some- 
times to the period immediately following. The objective or partially 
objective character of the modified ending is, therefore, not put in 
question by the variety of report. Moreover, as we have also shown 
above, prescribed direction of attention led to uniformity of report. 



324 bishop 

All tones were blown at full intensity, at the different durations. 
The O's sat where they could see the E's hand move as the tone was 
cut off. This movement replaced the signal flag of previous experi- 
ments. In all, 448 observations were made. Of these 11 were with 
15 min., and 7 with 5 min. stimulation. 

Results 

Not an after-image was reported; but our practised O's 
heard modified endings, and felt pressure in the ear from long 
stimulation. 

Our regular experiments had indicated that pressure in the 
ears made its appearance in place of an after-image, and we 
wanted striking evidence that this indication was true. If 
naive O's could be brought to sense the pressure, without prac- 
tice, this result would be good evidence of its fundamental 
nature. Accordingly, our unpractised O's were instructed 
to describe any after-effect of stimulation that might appear. 
They were then given number 2 on the reedbox as stimulus 
for 15 min. All of the group, except one who misunderstood 
the instruction, reported vivid, throbbing pressure which lasted 
from 5 to 10 min. after stimulation. Nothing was heard. 
One O remarked that there was no after-image, unless the 
throbbing were a negative after-image ; and added that in this 
condition the ears " felt very sensitive to noise." If 15 min. 
of stimulation resulted in so perceptible a pressure, it may 
be supposed that the naive O's who were able to report it 
were also capable of perceiving any tonal after-effect, and 
that their failure to perceive tone is further evidence that no 
auditory after-image exists. The case of heightened sensi- 
tivity to noise may help to explain why, in the regular series 
of experiments, long stimuli led to more frequent report of 
modified endings. 

Auditory Recurrent Images 

For T, the tone frequently recurred after stimulation. S 
reported only a single recurrence, the other O's none. The 
attributive correlates of T's recurrences are given in Table X. 

The recurrences were generally 1 to 2 sec. in length, but 
sometimes were as long as 10 sec. More than a single recur- 
rence was not reported after the sources of tone had been 
placed in sound-proof boxes. There were numerous ' recur- 
rences,' as many as seven, when the purr of the motor could 
be heard through the conduction-tube. These recurrences are, 
of course, not trustworthy. As a rule, the recurrent tone had 
the same pitch as the stimulus ; but it was sometimes one 
or two octaves above or below. Localization was generally 



THE POSITIVE AFTER-IMAGE IN AUDITION 325 

in the head, though sometimes recurrences were unlocalized. 
B found that, after serving as E for one or two hours, the 
recurrent tone came as an addition to any faint continuous 
objective sound, but that in a quiet room no recurrent image 
could be heard. Our recurrent images obviously resemble 
the " secondary " after-sensations of Urbantschitsch. 











TABLE X 














Intensity 




Duration 




Pitch 




Series 


Str. 


Med. 


Wk. 


30 


15 


5 


1024 


512 256 


128 


I 


10 


11 


3 


8 


9 


7 


1 


8 11 


4 


II 





1 


1 


2 











1 


1 


III 


8 


3 





3 


5 


3 


5 

435 


3 
350 256 


3 
100 


IV 


5 


1 


1 


4 


2 


1 


2 


1 4 





Total 


23 


16 


5 


17 


16 


11 









In Tables III and IV, T's results are often not in agree- 
ment with those of the other O's ; but it will now be seen that 
the disagreement is due to the number of recurrences at higher 
and middle intensities, or in long and middle durations. The 
unequal distribution for pitch in Table X may be due, in 
part, to the influence of the register of T's voice; the recur- 
rences are most frequent within his singing range. 

Conclusions 

1. There is no positive after-image of tone, analogous to 
the positive after-image of vision. 

2. The ' modified ending ' of tones, which we have been 
unable to eliminate, is probably a compound effect, due in 
part to tonal Abklingen, in larger part to the objective con- 
ditions of our experimental arrangement. 

3. Intensity of modified ending is dependent upon intensity 
of stimulus ; its vividness or insistence upon pressure in the 
ear, which increases with intensity and duration of stimulation. 

4. It is impossible from our experience to say which of our 
two sources of tone, the variators with air conduction or 
telephone receivers, is to be preferred. The variator tones 
were relatively weak ; when they are cut off, slight noise accom- 
panies the swinging of the conduction-tube ; if the conduction- 
tube remains unbroken, echo is a source of error. They 
furnish, however, an almost purely tonal Abklingen. The 
telephone receivers give tones approximately equal in intensity 
at all pitches ; in the period when the after-image is expected 
to appear, they are absolutely noiseless ; there is no difficulty 
from echo; but the Abklingen is very impure, owing to an 
admixture of noise from the diaphragm. 



